1. Field of the Invention
The present invention relates to a method for driving a solid-state image sensor.
2. Description of the Prior Art
So far a great deal of research-and-development work on various solid-state image sensors has been made so that their performance is now almost equal to or even superior to the conventional single-tube image sensors.
In general, a solid-state image sensor comprises an array of photosensors which converts a pattern of light focused thereupon into a pattern of charges which in turn are stored therein, and a scanning section or stage for scanning the photosensor array, thereby moving the resulting packs of charge to an output terminal.
A photosensor array consists of, for instance, an array of m.times.n photodiodes each of which is associated with a MOS transistor switch for transferring signal charge onto a vertical signal line. A vertical shift register is provided so that all the MOS transistor switches in each column are turned on and off and when the MOS transistor switch is turned on, it establishes an electrical connection between the associated photodiode and the vertical signal line. The vertical signal lines are connected to a horizontal readout circuit which is adapted to scan the vertical signal lines one column at a time so that the signal charges or packs of charge can be serially transferred onto the horizontal signal output line.
The so-called solid-state image sensor comprises, in general, a horizontal readout circuit consisting of a horizontal shift register for generating the horizontal scanning pulses and an array of switching transistors which are interconnected between the vertical signal lines and the horizontal output signal line and which is controlled by the horizontal shift register. However, the solid-state image sensor of the type just described above has a drawback that noise produced when the horizontal switching transistors are turned on and off results in noise in the form of fixed patterns on the reproduced picture, whereby the reproduced picture quality is greatly degraded.
In order to overcome this drawback, it has been proposed to use a charge-coupled device (to be referred to as "CCD" for brevity in this specification) instead of the above-described type horizontal readout or scanning circuit.
One of the most simple solid-state image sensors of the type employing CCDs is such that the vertical signal lines are connected through transfer gates to a horizontal readout CCD (to be referred to as "the horizontal CCD" for brevity in this specification). With this arrangement, however, one end of each vertical signal line must be connected to an n-type diffused region of a pn junction so that the signal charge on the vertical signal line can be injected into the horizontal CCD. However, since the interior of the diffused region is not influenced by the electric fields, it becomes difficult to transfer the signal charge from the diffused region to other potential wells within a brief time period in an efficient manner. As a consequence it is almost impossible to transfer the signal charge on the vertical signal line to the region immediately below the gate electrode of the horizontal CCD with a transfer efficiency higher than 90% within one third or a quarter of the horizontal blanking period T.sub.BLK (about 16.5 microseconds). Therefore, the solid-state image sensor of the type described above has not been successfully used in practice.
In order to overcome the above-described problem, it has been proposed to make an improvement of the transfer stage or section between the vertical signal lines and the horizontal CCD. The transfer section or stage has two gates and one charge storage means for each vertical line so that the signal charge which has been transferred onto the vertical signal line can be efficiently transferred into the horizontal CCD. More specifically, a first gate is turned on so that the charge of a predetermined value (to be referred to as "the internal bias charge Q.sub.p " in this specification) is transferred from the charge storage means onto the vertical signal line and added to the signal charge Q.sub.S transferred from the photodiode. Thereafter the signal and internal bias charges Q.sub.S and Q.sub.p are returned back to the associated charge storage means and the first gate is turned off. Next, the second gate is turned on, and the signal charge Q.sub.S is transferred into the horizontal CCD. Because of such improvement of the transfer section or stage as described above, the solid-state image sensors employing CCDs have come to be used in practice, but there is still a problem to be solved. That is, only the signal charge Q.sub.S is transferred from the charge storage means into the horizontal CCD so that if the quantity of the signal charge Q.sub.S is too small, the transfer efficiency is considerably reduced. The degradation in transfer efficiency in turn will result in the degradation of color-separation characteristics with the resultant color mixturing and the like. Moreover, in response to the intensity of the incident light, variations of hue in color occur.
Meanwhile, a small-sized solid-state image sensor of the type employing a bucket bridge device (BBD) as a horizontal scanning circuit is reported in IEE Journal of Solid-State Circuits, Vol. SC-15, No. 2, pp 206-213. In this image sensor, the bias charge is transferred from the BBD to a vertical signal line and added to the signal charge of a small value generated when the intensity of the incident light is low. Thereafter the signal charge plus the bias charge are transferred back into the BBD, thereby attempting to increase the charge transfer efficiency. The "buckets" of the BBD consist of a diffused region or layer so that the control on the voltage on the vertical signal line and on the voltage at the associated "bucket" for the transfer of the charge becomes easy. However, in the case of the conventional television system, about 400 horizontal shift registers must be provided so that the horizontal scanning frequency becomes higher than 7 MHz and consequently the horizontal transfer frequency becomes also higher than 7 MHz. With the BBD, however, it becomes impossible to attain a charge transfer efficiency higher than 99.99% through each of the horizontal shift registers at such high frequency. In addition, the horizontal shift register cannot accomplish the charge transfer operation in response to the continuous clock pulses. As a consequence, in order to otain higher transfer efficiency of signal charge from a vertical signal line into a horizontal shift register, a time period as long as permissible within the horizontal blanking period must be used and during this time period the horizontal shift register must be halted. It follows that it becomes difficult to uniformly inject the signal charges into the stages of the horizontal BBD shift register. As a result, horizontal shading tends to appear in a reproduced television picture over tens of bits from the trailing edge of the horizontal scanning.
Therefore, because of the relationship between the charge transfer speed and the charge transfer efficiency in practical solid-state image sensors, BBD horizontal shift registers cannot be used, but CCD horizontal shift registers must be used.